1. Field of the Invention
The present invention generally relates to an automatic focus detecting device having a focus detecting area and, more particularly, to the automatic focus detecting device which can be practiced in a single lens reflex camera or a video camera having an AF function of accomplishing an automatic focusing by discriminating a object to be photographed or videoed within the field of view.
2. Description of the Prior Art
Hitherto, an automatic focus adjusting device is widely used which is operable to move the photo-taking lens to an infocus position on the basis of a result of focus detection carried out by a focus detecting means. In such automatic focus adjusting device, when the photo-taking lens is moved to the infinity position, where the lens is focused on an object at infinity, it may happen that the focus detection of an object close to the camera cannot be achieved. In view of this, the use of a limit switch has been made to detect the arrival of the photo-taking lens at the infinity position or the closest position so that, in the event that the focus detection is disabled and it is detected the arrival of the lens at the infinity position or the closest position, the photo-taking lens can be moved in a reverse direction to search for the object which can be automatically focused. This is disclosed in, for example, the Japanese Laid-open Patent Publication No. 58-224318 published in 1983.
Also, an attempt has been proposed in, for example, the Japanese Laid-open Patent Publication No. 59-48719 published in 1984 wherein, even though the photo-taking lens is not held at the infinity position or the closest position, in the event of the incapability of the focus detection, the photo-taking lens can be moved so that the direction of movement of the lens can be determined depending on the position at which the photo-taking lens is brought to a halt.
In the meantime, depending on the type of and the focal length of the lens, no focus detection is possible with respect to a main object occupying a position close to the camera. This is particularly true when the photo-taking lens is moved to the infinity position. In such case, the following problem will arise. Assuming that the main object and other objects distant from the main object are found within the focus detecting area, and if the photo-taking lens is moved to the infinity position at this time, focus detection is possible with respect to such other objects while the focus detection is impossible with respect to the main object.